1. Field of the Invention
The present invention relates to a common rail fuel injection control device applied to diesel engines, more specifically to a device, which executes feedback control of common rail pressure, for converting the actual common rail pressure into values suitable for control, and to a method therefor.
2. Description of the Related Art
Common rail fuel injection control devices for diesel engines in which a common rail pressure is feedback controlled for optimizing the injection pressure according to the operation state of the engine, such as revolution speed and load, are well known.
In such a feedback control, the control is conducted so as to match the actual common rail pressure with a target common rail pressure determined based on the engine operation state. More specifically, the control is executed based on the difference between those pressures. Accordingly, the detection of the actual common rail pressure with a pressure sensor has been carried out. Typically, in this control, the values detected by the pressure sensor are directly used as representative values of the actual common rail pressure (for example, Japanese Patent Application Laid-open No. H11-30150 (paragraph 0018), Japanese Patent Application Laid-open No. S63-50649 (page 5), and Japanese Patent Application Laid-open No. 2000-257478 (page 5)).
Because fuel supply into a common rail is conducted by a supply pump pumping the fuel within the prescribed periods, pulsations caused by pumping with the supply pump occur in the actual common rail pressure. Those pressure pulsations are shown with the diagram denoted by “Real Rail Pressure” in FIG. 1 and the diagram denoted by “Actual Pressure (Related Art)” in FIG. 2. FIG. 1 is shown on a macro scale in FIG. 2.
As shown in FIG. 1, in this example, fuel pumping with the supply pump is conducted in ΔT=180 CA (180° crank angle, same hereinbelow) periods, and the control period of the control device is Δt=30 CA (⅙ of the pump pumping cycle ΔT). As shown by black dots, the values detected by the pressure sensor (sensor detected values) are read by a controller every control period Δt. The control is usually conducted by using the sensor detected values as the representative values of the actual common rail pressure.
However, the sensor detected values also greatly fluctuate according to pulsations of the actual common rail pressure. Therefore, in the feedback control, especially the PID control, the difference between the target value and actual value and also the values of the proportional term and differential term determined based on this difference always vary significantly. As a result, directly using the sensor detected values create a risk of degrading the controllability.
The diagram denoted by “Differential Term (Related Art)” in FIG. 2 is a differential term calculated by using the sensor detected values. This figure demonstrates that the differential term constantly changes, and using the value thereof is clearly undesirable.
When conducting control by using such fluctuating sensor detected values, setting a feedback control gain to a comparatively small value can be considered. However, such an approach degrades the responsiveness of the feedback control.
Accordingly, filtering processing conducted to average a plurality of sensor detected values obtained within the prescribed interval can be considered. The problems are, however, that setting the averaging interval is inappropriate: when it is too long, it causes a response delay, and when it is too short, the fluctuations cannot be completely eliminated.